This invention relates to combustion control apparatus for an internal combustion engine and particularly to the use of peak combustion pressure for ignition timing control and the control of combustible mixture composition, particularly A/F ratio, for cylinder torque balancing at the lean limit of engine operation.
Location of peak pressure (LPP) ignition controls are now well known in the patent and publication prior art and are regularly used in the calibration of production engines. In such controls, the location, relative to a reference crankshaft rotational angle such as TDC, of peak or maximum combustion pressure (LPP) is sensed; and ignition timing is controlled in closed loop operation to maintain LPP at a predetermined angle, which may be constant or may vary slightly with engine speed. Patents showing such systems include Laubenstein et al U.S. Pat. No. 4,153,019, issued May 8, 1979, Brandt et al U.S. Pat. No. 4,406,265, issued Sept. 27, 1983, Karau et al U.S. Pat. No. 4,481,925, issued Nov. 13, 1984 and Fattic et al U.S. Pat. No. 4,601,197, issued July 22, 1986, all of which issued to the assignee of this invention. Other patents showing such systems include Peterson U.S. Pat. No. 3,957,023, issued May 18, 1976 and Powell et al U.S. Pat. No. 4,063,538, issued Dec. 20, 1977.
It has also been suggested that the location of peak combustion pressure LPP may be applied to the control of air/fuel ratio as well as ignition timing. The U.S. Pat. No. 4,391,248 to Latsch, issued July 5, 1983, discloses a system for controlling combustible mixture composition in response to LPP. The U.S. Pat. No. 4,561,401 to Hata et al, issued Dec. 31, 1985, discloses a system for controlling combustible mixture composition in response to LPP variations from a predetermined range.
It has further been shown, in the U.S. Pat. No. 3,977,373 to Sand, issued Aug. 31, 1976, that ignition timing or a combustible mixture composition affecting parameter such as EGR may be varied in response to the value or magnitude of peak combustion pressure in order to maintain it within predetermined limits.
It is known that, at the lean limit of engine operation, variations in the mixture of the combustible charge supplied to each combustion chamber may cause one or more of the chambers to be beyond the lean limit and thus lead to undesirable combustion or misfire. Various systems have been suggested for sensing a torque indicating parameter associated with the firing of each combustion chamber, comparing the value of this parameter with an average value for all combustion chambers and adjusting the mixture of the combustion chamber if it is too different from the average. One such parameter is derived from combustion pressure measured after combustion is complete, as described in the U.S. Pat. No. 4,621,603 to Matekunas, issued Nov. 11, 1986.
However, the Matekunas approach is based on a plurality of pressure measurements at predetermined crankshaft angles and requires a great deal of computer calculation time to derive the required values. It does not use either the location or magnitude of maximum combustion pressure and, in fact, includes no means for detecting the maximum. The goal of this invention is, rather, to add fuel control capability for torque pulse evening to an LPP ignition timing system, wherein only one combustion pressure (peak) is sampled, to optimize engine operation at the lean limit.